Various methods have been proposed in the past for detecting gases in gaseous mixtures. Faraday showed in the last century that all matter is magnetic. Matter strongly attracted to a magnetic field was termed paramagnetic and matter repulsed by a magnetic field was termed diamagnetic. Few gases have strong paramagnetic properties, amongst those with the strongest are oxygen O.sub.2, nitrogen dioxide NO.sub.2 and nitrous oxide NO.
This property exhibited by only a few gases has been used in the past for the determination of the proportion of a paramagnetic gas, usually oxygen, in a gas mixture using generally four methods:
1. Magnetic Wind. This method suffers from effects such as thermal conductivity of background gases, low sensitivity, long response time and is not used much nowadays.
2. Differential Pressure. This method requires a reference gas and in some forms requires an alternating magnetic field. It can be made to have a somewhat fast response, but the smallness of the differential pressure created requires very sensitive flow sensors.
3. Magnetic Auto-balance. A torsion balance with automatic feed back control via an optical system. The most popular method, but suffers mainly from slow response time, and gas flow errors.
4. Relative Change in Permeability. Many different approaches to this method have been attempted without a great deal of success due to the extreme smallness of the resulting change in permeability.
Thus there remains a need for a method and apparatus for the determination of a proportion of paramagnetic gas in a mixture of gases which is relatively simple to use and has a quick response to changes in the gas mixture being measured.